One of the operations which is required when drilling a horizontal drainhole is the sidetracking process wherein drilling from an existing cased wellbore into the adjacent formation is initiated. Currently, there are several different ways that this can be done, none of which is entirely satisfactory.
One method to initiate sidetracking is to fill the casing with cement to the desired depth, which will force the drilling equipment into the formation. This is possible if the cement has a higher compressive strength than that of the formation. This method is unsatisfactory, however, since the cement plug does not always have a compressive strength greater than the formation and accurate control of the hole direction is difficult.
Another method is to run a whipstock into the cased wellbore on an inflatable packer to the desired depth, after which an orientation survey is run, the whipstock face is turned in the desired direction and the packer is inflated. While this method can properly orient the whipstock initially, it is difficult to maintain the desired direction because the packer tends to turn as it is inflated, causing the whipstock face to turn with it. In addition, the packer is subjected to torque forces caused by the drilling operation. If the torque is severe enough the friction between the inflated element and the casing is overcome, causing the assembly to turn or drop down the casing. There is also the possibility that the packing element could deflate and drop down the casing.
A third method is to set a permanent seal bore packer with a keyed sub at the desired depth in the casing, run an orientation survey to determine the orientation of the key and set the angle between the key slot of the whipstock anchor and the whipstock face. The whipstock anchor is then run into the packer and latched in place, which ensures that the whipstock face is set in the desired direction. Although the equipment used in this method is designed to enable the whipstock and anchor to be withdrawn while leaving the packer in place, it has been found that as the formation is being drilled, cuttings tend to settle out on top of the packer and in the annulus between the whipstock and casing, which interferes with the removal of the whipstock assembly and makes the entire system permanent. Since a whipstock/packer assembly is costly, the ability to retrieve and reuse not only the whipstock assembly but the packer as well would greatly reduce expenses for the drilling of a horizontal drainhole.
It is therefore an object of the invention to provide a whipstock/packer assembly which is capable of being precisely positioned in the intended direction of the drainhole to be drilled and which can readily be retrieved from a well after the hole is drilled.